Throughout this specification the use of the word “inventor” in singular form may be taken as reference to one (singular) or more (plural) inventors of the present invention. The inventor has identified the following related art.
In Applicant's co-pending International (PCT) Applications, PCT/AU03/00502 and PCT/2004/001036 both published under WIPO publication Numbers WO 03/094037 and WO 2005/11128 respectively, a number of background art systems have been identified relating in particular to wireless communications systems based on so-called multiple access techniques in which information such as voice and data are communicated. The specifications of WO 03/094037 and WO 2005/11128 are incorporated herein by reference in their entirety.
The inventor has recognised that the performance of a Mobile Communications System may be heavily dependent on the quality of the Physical Layer (PHY) processing. The PHY may provide for delivering coverage and robustness to radio links between nodes that move through hostile propagation conditions such as those encountered outdoors, and areas of high interference. Mobility, and in particular high speed terrestrial mobility, may induce yet another set of difficulties for the PHY as the reflections off the surrounding buildings, vehicles and other bodies may combine in a time varying manner.
Some wireless network vendors have incorporated legacy IEEE 802.11 radio technologies into their systems. Conventional IEEE 802.11 radios have been designed for stationary indoor propagation environments and their use in outdoor mobile communications networks may be ill founded from a technical perspective. Standardization efforts within the IEEE 802.16e and 802.20 Physical Layer working groups may be considered as focused on providing a waveform for transmission that is more compatible with the communications challenges faced while travelling outdoors at speed. Standards typically do not specify how to receive signals, rather focussing on what signals should be transmitted. The vendors are then responsible for the receiver technology.
The inventor considers that Orthogonal Frequency Division Multiplexing (OFDM) is well suited to broadband wireless communications. However, this technique may have been historically applied to the problem of transmitting data in a stationary indoor environment. The outdoor urban environment may contain many obstacles for the radio signal, such as buildings and trees, which are referred to as clutter. Present wireless technology may be able to offer high throughput only at the expense of receiver sensitivity, hence the cluttered urban environment may lead to poor coverage. Furthermore, the relative mobility between the transmitter and receiver may cause the placement of these obstacles to change in time. When the effects of mobility and clutter combine, the resulting wireless channel may present a significant challenge to the communications system designer.
The radio signal in an outdoor communications system may be subject to distortion caused by the propagation environment, i.e. channel, on the radio signal. The channel may distort the transmitted signal by altering its magnitude and/or phase, potentially resulting in the loss of information. Moreover, relative mobility between the transmitter and receiver, and/or time varying frequency offset effects, cause the channel conditions to vary with time.
Signal reflections and diffractions can result in multiple copies of the transmission being received, i.e. multipath effects. Typically each of these multipath components may have been subjected to different effects upon their phase and magnitude. The discrete time channel impulse response, and its associated Power Delay Profile (PDP), represent each multipath contribution as a time domain tap. A level of intensity, and a phase rotation, is assigned to each tap in order to represent its contribution to the overall received signal. The delay spread of the channel, is the delay between the arrival of the first and last multipath contributions in the PDP. The RMS delay spread, which is derived from its PDP, is a single value which accounts for each multipath contribution, weighted according to its delay and magnitude. A higher RMS delay spread indicates that the channel is likely to have a stronger effect on the signal.
The power delay profile for an example indoor wireless channel, based upon the ETSI BRAN model B [2] is shown in FIG. 1. This channel has an RMS delay spread of 150 ns.
Multipath propagation can lead to an OFDM symbol being subjected to interference from previously transmitted OFDM symbols, i.e. inter-symbol interference (ISI). Multipath effects can also degrade the orthogonality of subcarriers, thus leading to an individual subcarrier being subject to interference from other subcarriers within the same OFDM symbol, i.e. inter-carrier interference (ICI). OFDM provides several mechanisms which are intended to mitigate multipath effects. For example, it is common for each OFDM symbol to include a guard interval, which separates successively transmitted OFDM symbols. If the guard interval is selected to be greater than the delay spread of the channel then ISI and ICI cannot occur. The presence of this guard increases the time taken to transmit each OFDM symbol and thus reduces the data rate of the system, decreases the power efficiency and decreases the spectral efficiency of the system.
In order to successfully demodulate the signal, the influence of the channel is first cancelled and hence an estimate of the channel is required. A set of training symbols may be transmitted at the start of the packet to be used for channel estimation, which are known to the receiver. These symbols are referred to as preamble symbols and the technique is dictated in several standards, e.g. IEEE 802.11a/g and 802.16-2005. A common approach is to estimate the effect of the channel in the frequency domain [2]. To this end, a cyclic prefix 2 is inserted in the guard interval immediately prior to a symbol 4, containing a time-domain copy of the end 6 of the symbol, as shown in FIG. 2.
The presence of a cyclic prefix that is longer than the delay spread of the channel permits the assumption that the transmitted OFDM symbol undergoes a cyclic convolution with the channel. In this case, one method for calculating the channel response is to simply divide the received frequency domain symbol by the known preamble symbol. FIG. 3(a) shows an example of the channel estimate provided using this technique in an 802.11a system, for the indoor channel model presented above, in the absence of noise on the channel. The 802.11a standard specifies that each OFDM symbol have a total duration of 4 μs, including a 0.8 μs cyclic prefix. Moreover, the second long preamble symbol is identical to the first, providing an effective cyclic prefix of 3.2 μs duration for this symbol. This guard interval is significantly longer than the typical delay spread of the indoor channel and thus an accurate channel estimate can usually be obtained.
The coherence bandwidth of a channel is inversely proportional to its delay spread. Hence, as the indoor channel in the example above has a relatively short delay spread, a strong correlation between the frequency domain response of adjacent subcarriers is observed in FIG. 3(a). In cases where the coherence bandwidth is significantly larger then the subcarrier spacing, this correlation may be exploited to improve the accuracy of the estimate, for example by smoothing it across the frequency domain.
When the length of the delay spread does not exceed the cyclic prefix duration, the effect of the channel may be equalized in the frequency domain using a one-tap linear equalizer. Once the channel estimate has been obtained, it is used during the equalization process. Again employing the assumption of a time domain cyclic convolution of the transmitted data and the channel, the equalized frequency domain observation can be simply obtained via division of the received observation by the estimated channel response. FIG. 3(b) shows an observation, after frequency domain equalization, for the example indoor channel, when using an 802.11a system. Each OFDM data symbol is protected by a cyclic prefix of duration 0.8 μs, which is greater than the delay spread of the indoor channel, and hence no inter-symbol or inter-carrier interference is observed.
The outdoor propagation environment can be significantly more disruptive to a signal than that experienced indoors. When propagating through the outdoor urban environment, the radio signal is subject to obstacles such as buildings, trees and other clutter, which can lead to strong reflective and/or diffractive multipath effects [1]. As a result, the delay spread of the outdoor wireless channel is typically significantly larger than that of its indoor counterpart. The power delay profile for an example outdoor wireless channel is shown in FIG. 4. Here the IEEE 802.20 Typical Urban (Case-IV) model is employed, having an RMS delay spread of 0.8 μs.
The long delay spread experienced outdoors can present problems for conventional receiver techniques, such as the channel estimation and equalization methods described above. The problems arise if the cyclic prefix length is not sufficient to cover the delay spread of the channel. One such notable case exists when an OFDM waveform that is designed for indoor use, such as the IEEE 802.11a/g waveform, is employed in an urban outdoor environment. As described above, the second long preamble symbol in the 802.11a waveform is a replica of the first, and as such this training symbol is provided with an effective cyclic prefix of length 3.2 μs. Hence, when using the frequency domain channel estimation technique described above, the second long preamble is almost completely guarded from ISI and ICI effects being induced by the example outdoor channel. However, the residual channel effects cause a slight variation of the estimate from the exact channel, as illustrated in FIG. 5(a). Moreover, due to the long delay spread experienced outdoors, the coherence bandwidth of the channel is significantly shorter than that of the indoor channel. This may be observed when comparing the relatively low correlation between adjacent subcarriers in FIG. 5 (a) to the strong correlation shown in FIG. 2 (a). Hence the potential to improve the accuracy of the channel estimate via frequency domain smoothing is reduced for the case of the urban outdoor environment.
In contrast to the heavily guarded preamble, data bearing OFDM symbols in the 802.11a packet are only afforded a cyclic prefix guard of length 0.8 μs. Each multipath component which has a delay exceeding the guard length will contribute a part of the previously transmitted OFDM symbol into the received observation, effectively jumping over the guard interval. As a result, the assumption of a cyclic convolution between the time domain symbol and channel is invalid. The resulting equalized received observation is heavily distorted by ISI and ICI effects, as shown in FIG. 5(b).
The strong multipath effects experienced in an outdoor urban environment, result in a wireless channel which exhibits a high RMS delay spread. In some cases it may be possible to extend the length of the cyclic prefix beyond that of the delay spread. For example, the IEEE 802.16 standard provides several options for cyclic prefix length, one of which may be suitable for the particular radio channel being employed. However, an extension of the cyclic prefix results in wasted transmit energy and decreased spectral efficiency. In situations where transmit power is limited, e.g. by a regulatory body, wasted transmit energy equates to reduced range and/or data rate. In other cases the length of the cyclic prefix may be fixed, e.g. IEEE 802.11a/g, and conventional receiver techniques may fail under stress of the resulting inter-symbol and inter-carrier interference.
Any discussion of documents, devices, acts or knowledge in this specification, either within the text of this specification or, material incorporated herein by reference is included to explain the context of the invention. It should not be taken as an admission that any of the material forms a part of the prior art base or the common general knowledge in the relevant art in Australia, the United States of America or elsewhere on or before the priority date of the disclosure and claims herein.